Our research during the last three years has been extremely rewarding. By combining techniques in High Energy Physics (multiwire Proportional Chamber) with the state-of-the-art in Digital Electronics (Mass Core Memory, Video Display, etc.) we were able to set up a high speed computer controlled diffractometer for protein crystallography. This first system, called MARK I, can collect data about 10 times faster than a standard diffractometer. It has been used to collect more than a million intensity measurements with six different types of protein crystal. The precision of the data is good and with R intensity factor varying from 4% to 7%. We have also started the construction of a MARK II system using up to six chambers as detectors and a rotating anode device as an x-ray generator. This proposal is a requent for continuation of financial support to finish the construction of the MARK II system which will be able to collect data efficiently for protein crystals with unit cell dimension as high as 400 A and will have a data collection rate of about 100 times faster than the diffractometer. We also want to use this system to collect data to solve the structure of new and interesting proteins (chicken liver dihydrofolate reductase and cytochrome c peroxidase in collaboration with Professor Kraut's group at UCSD, and cytochrome c' in collaboration with Professor Salemme's group at the University of Arizona). We also plan to use the unique quality of this system to do experiments that cannot be done anywhere else.